The present invention relates to a heat exchanger brazed in a brazing furnace, comprising stacked heat exchanger plates which between them have ducts for different media, and braze material in the brazed connection seams. The invention also relates to a method for producing the same.
The type of brazed heat exchanger used herein as a basis for the introduction of improvements is, for example, known from EP 1 152 204 B1. Few details are given in that document with regard to the brazing, the type of braze alloys used, or the brazing process.
The material of the heat exchanger plates and the material of the fins arranged in the ducts are often a suitable steel, in some cases a high-grade steel.
With regard to braze materials suitable for the brazing of steel, there are, for example, Ni-based braze materials, Cu-based braze materials, Fe-based braze materials, Cr-based braze materials, and the like. In a known manner, the expression “based” expresses that the respective metal constitutes the main alloy constituent, or at least a major constituent of the braze material, in conjunction with other alloy constituents such as Mn, Mo, P and Si, to name but a few.
From DE 100 03 329 B4, for example, it is known to use two different Cr-based braze materials for the brazing of an exhaust-gas recirculation cooler. Said braze materials are either mixed before being applied, with the mixture applied to individual parts of the cooler to be brazed or to a base material, or the two braze materials are applied one on top of the other as a lower and upper layer in the form of a braze paste. Improved corrosion resistance of the exhaust-gas cooler can be achieved with such a proposal.
In EP 1 153 690 A1, to achieve the same aim, the base material is plated with three braze materials having different base substances, specifically first a Cr-based braze material, then a Ni-based braze material and finally a Cu-based braze material. The same braze materials are provided in all of the ducts of said heat exchanger, which is expedient inter alia from a manufacturing and logistical aspect. In the EP document, the use of braze pastes, braze powders or braze foils is regarded as being disadvantageous. Electrical or non-electrical plating methods are thus performed on the base material of the heat exchanger using the braze materials.
A method for producing brazed plate-type heat exchangers is known from DE 103 28 274 A1. The method provides a combination of particular welding and brazing working steps. The use of braze foils or the application of braze pastes or braze powders is also mentioned. The document also claims a number of suitable braze materials.
Finally, it is also known from practice for the two flat sides of an aluminum sheet-metal strip from which heat exchanger plates are produced to be plated with different Al-based braze materials. The braze materials are typically Al—Si-based braze materials. They differ, however, with regard to the content of certain alloy constituents. In the heat exchanger, the plates are arranged such that identical braze materials are provided on the surfaces of all of the ducts of the heat exchanger.